Radio frequency (RF) filtering circuitry is essential to the operation of modern wireless communications devices. In addition to reducing distortion present in RF transmit and receive signals, RF filtering circuitry is also used to separate and combine RF signals in different frequency bands such that multiple RF signals can be simultaneously transmitted and received from a single antenna. One type of RF filtering circuitry often used for this task is RF multiplexer circuitry. FIG. 1 shows a conventional RF multiplexer 10. The conventional RF multiplexer 10 includes a common node 12, a number of input/output nodes 14, and RF filtering circuitry 16 coupled between the common node 12 and the input/output nodes 14. Generally, the common node 12 of the conventional RF multiplexer 10 is coupled to an antenna, while each one of the input/output nodes 14 are coupled to RF front end circuitry. RF receive signals from the antenna are provided at the common node 12, where they are separated by the RF filtering circuitry 16 and passed to one of the input/output nodes 14. Each one of the input/output nodes 14 is associated with a particular frequency band such that each one of the input/output nodes 14 receives only the portion of the RF receive signals falling within their associated frequency band from the common node 12. This allows RF receive signals to be routed to low noise amplifier (LNA) circuitry in the RF front end circuitry that is optimized to operate within the particular frequency band, which improves the performance thereof. RF transmit signals provided from the RF front end circuitry to each one of the input/output nodes 14 are combined and passed to the common node 12, where they can then be transmitted from the antenna.
Generally, RF filtering circuitry can be divided into two categories:
acoustic filtering circuitry and electromagnetic filtering circuitry (that can be either lumped or distributed). FIG. 2 shows the conventional RF multiplexer 10 wherein the RF filtering circuitry 16 is acoustic filtering circuitry 18. Specifically, the RF filtering circuitry 16 is shown as a bulk acoustic wave (BAW) or a surface acoustic wave (SAW) filter.
To implement the conventional RF multiplexer 10, the acoustic filtering circuitry 18 must provide a filter response configured to pass the particular frequency bands associated with each input/output node 14. For the RF multiplexer 10 shown in FIG. 1, this means providing a filter response with a bandpass for six different frequency bands. Due to size constraints associated with mobile devices, the acoustic filtering circuitry 18 is generally implemented on a single module. As the number of desired pass bands gets larger and stop bands are needed to ensure good isolation between bands, the complexity of the acoustic filtering circuitry 18 increases substantially. The number of frequency bands that can be separated by conventional acoustic circuitry 18 is therefore limited.
Acoustic filtering circuitry generally has a relatively high quality factor (Q), however, the bandwidth thereof is generally limited. This is due to the fact that as the bandwidth of acoustic filtering circuitry is increased, an undesirable filter response known as “flyback” is also increased. To illustrate this effect, FIG. 3 is a graph showing an exemplary portion of a filter response of acoustic filtering circuitry. As shown in FIG. 3, the acoustic filtering circuitry exhibits a bandpass response 20 with very steep roll-off (due to the high quality factor thereof). However, even for a relatively narrow bandpass response, the filter response shows significant flyback 22. This amount of flyback is generally proportional to the number of acoustic filters sharing a common node. The flyback associated with acoustic filtering circuitry with a large number of filter responses therefore generally limits the achievable bandwidth thereof. As wireless communications standards continue to incorporate additional bands that span across a wide range of frequencies, and as carrier aggregation configurations in which wireless communications circuitry is required to simultaneously utilize these different bands, become more widely accepted, the constrained bandwidth of acoustic filtering circuitry has become problematic.
Electromagnetic filtering circuitry has thus been used to achieve wider bandwidths in wireless communications circuitry. FIG. 4 shows the conventional RF multiplexer 10 wherein the RF filtering circuitry 16 is electromagnetic in the form of lumped LC filtering circuitry 24. The LC filtering circuitry 24 includes a number of inductors 26 and a number of capacitors 28 coupled between the common node 12 and the input/output nodes 14. While the LC filtering circuitry 24 provides a much wider bandwidth than the acoustic filtering circuitry 18, a wide bandwidth is generally associated with higher loading along the signal path of the filtering circuitry. This generally results in a significant reduction in the quality factor of the LC filtering circuitry 24. Such a reduction in quality factor can result in relatively poor isolation between adjacent bands to be separated by the LC filtering circuitry. This is illustrated in FIG. 5. FIG. 5 shows a first bandpass filter response 30 and a second bandpass filter response 32 that are adjacent to one another. Within each bandpass filter response, a number of sub-bands are shown, labeled A through N. While each one of the first bandpass filter response 30 and the second bandpass filter response 32 has a relatively wide bandwidth, due to the relatively low quality factor associated with active filtering devices, the roll-off of each bandpass filter response is quite gradual. This results in significant overlap between the first bandpass filter response 30 and the second bandpass filter response 32, which translates to poor isolation between the pass bands. Specifically, this overlap often results in leakage of undesirable signals into a signal path in wireless communications circuitry, which can cause distortion, desensitization, and even damage to one or more components therein. This may be especially problematic in carrier aggregation applications in which two bordering sub-bands from each filter response are used. For example, in a carrier aggregation application in which signals are transmitted on band G and received on band H, a significant portion of the band G signals will likely leak into the signal path for band H, and vice versa.
In light of the above, there is a need for RF filtering circuitry with improved performance. Specifically, there is a need for RF filtering circuitry with improvements to the quality factor and bandwidth thereof.